1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and a method for fabricating the same, and more particularly, to an LCD device that can simplify a fabricating process, and a method for fabricating the same.
2. Discussion of the Related Art
With rapid development of information technology, flat panel display devices are in great demand because of their superior characteristics such as slimness, light weight, and low power consumption. Among those flat panel display devices, liquid crystal display (LCD) devices having excellent color reproduction are being aggressively researched and developed.
An LCD device mainly includes a top substrate, a bottom substrate, and a liquid crystal layer interposed therebetween. The top substrate and the bottom substrate are arranged to face each other. There are a plurality of electrodes formed on facing surfaces of the two substrates. The liquid crystal layer is usually formed by injecting liquid crystals between the two substrates. When a predetermined voltage is applied between the two substrates, an electric field is generated to align liquid crystal molecules in one direction, thereby varying light transmittance. In this way, the LCD device displays an image according to the varying light transmittance.
As one example of various types of LCD devices, an active matrix LCD (AM-LCD) device is widely used because of its excellent resolution and moving picture reproduction. In the AM-LCD device, thin film transistors (TFTs) and pixel electrodes connected thereto are arranged in a matrix form. Also, the pixel electrodes are formed on a bottom substrate (an array substrate), and common electrodes are formed on a top substrate (a color filter substrate). Liquid crystal molecules are driven by an electric field perpendicular to the substrates. The AM-LCD device has high transmittance and aperture ratio. Moreover, since the common electrodes of the top substrate may serve as ground, it is possible to prevent liquid crystal cells from being damaged by static electricity. The top substrate of the LCD device further includes a black matrix for preventing light leakage at a portion other than the pixel electrodes. The bottom substrate of the LCD device is formed by repetitively performing a thin film deposition and a photolithography using a mask. Typically, the LCD device is fabricated using four or five masks. The number of masks used represents the number of processes of fabricating the array substrate.
The array substrate also includes a gate line and a data line that may be formed of a conductive metal such as chromium (Cr), molybdenum (Mo), and tantalum (Ta). Since the conductive metal has superior thermal stability, the formation of hillocks can be prevented. The conductive metal is deposited on the substrate using a physical vapor deposition (PVD) process (e.g., sputtering) and is wet-etched or dry-etched to form the data line and the gate line. Although the conductive metal has the superior thermal stability, high resistivity of the conductive metal may cause a signal delay when the screen size of an image display device becomes larger. Therefore, materials that have low resistivity and do not cause the hillock are essential to the fabrication of the image display device.
New materials for the lines are required to fabricate image display devices of 15 inches or more, super extended graphics array (SXGA) display devices, and ultra extended graphics array (UXGA) display devices. Copper (Cu) and aluminum (Al) are recommended as the adequate line materials because their resistivity is the lowest. However, the aluminum may cause hillocks, and thus aluminum alloy with high resistivity is proposed as an alternative material. Moreover, studies are being conducted on the use of copper (Cu) as a low-resistivity line material.
Hereinafter, an array substrate of an LCD device and a method for fabricating the same according to the related art will be described with reference to FIGS. 1 and 2. FIG. 1 is a plan view schematically illustrating a related art array substrate of an LCD device, and FIG. 2 is a cross-sectional view taken along line I-I′ of FIG. 1. Referring to FIGS. 1 and 2, an array substrate includes a gate line 121 formed in a horizontal direction and a gate electrode 122 extending from the gate line 121. A first diffusion barrier layer 123 is formed under the gate line 121 and the gate electrode 122. The first diffusion barrier layer 123 enhances adhesion between a gate metal layer and an insulating substrate 110 during a gate metal deposition process. A gate insulating layer 130 is formed on the gate line 121 and the gate electrode 122. An active layer 141 and ohmic contact layers 151 and 152 are sequentially formed on the gate insulating layer 130. A data line 161, a source electrode 162, a drain electrode 163, and a capacitor electrode 165 are formed on the ohmic contact layers 151 and 152. Specifically, the data line 161 is formed perpendicular to the gate line 121, and the source electrode 162 is formed extending from the data line 161. The drain electrode 163 is formed opposite to the source electrode 162 with respect to the gate electrode 122. The capacitor electrode 165 is formed to overlap the gate line 121.
A second diffusion barrier layer 164 is formed under the data line 161, the capacity electrode 165, and the source and drain electrodes 162 and 163. The second diffusion barrier layer 164 prevents metal ions of the data line 161 from being diffused into adjacent layers. The data line 161, the source and drain electrodes 162 and 163, and the capacity electrode 165 are covered with a passivation layer 170. The passivation layer 170 has a first contact hole 171 exposing the drain electrode 163 and a second contact hole 172 exposing the capacity electrode 165. A pixel electrode 181 is formed on the passivation layer 170 disposed in a pixel region defined by the crossing of the gate line 121 and the data line 161. The pixel electrode 181 is connected to the drain electrode 163 and the capacitor electrode 165 through the first and second contact holes 171 and 172, respectively. In this way, the array substrate of the LCD device is fabricated by performing a photolithography process using a plurality of masks and a sputtering deposition process several times.
However, the photolithography process includes a plurality of processes such as a cleaning process, a coating process, an exposing process, a developing process, and an etching process. Also, a sputtering deposition process must be performed in a separate sputtering chamber. If one photolithography process and/or one sputtering process can be omitted, fabrication time and fabrication cost will be reduced, and a failure rate of the LCD device also will be decreased. For this reason, it is important to reduce the number of masks and sputtering deposition processes for fabricating the array substrate of the LCD.